Method of forming white appearing anodized films by laser beam treatment

ABSTRACT

The embodiments described herein relate to forming anodized films that have a white appearance. In some embodiments, an anodized film having pores with light diffusing pore walls created by varying the current density during an anodizing process is described. In some embodiments, an anodized film having light diffusing micro-cracks created by a laser cracking procedure is described. In some embodiments, a sputtered layer of light diffusing aluminum is provided below an anodized film. In some embodiments, light diffusing particles are infused within openings of an anodized layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a 35 U.S.C. §371 national phase entry of PCT/US2013/047163 filedJun. 21, 2013 entitled “WHITE APPEARING ANODIZED FILMS AND METHODS FORFORMING THE SAME,” which claims priority to U.S. Provisional ApplicationSer. No. 61/663,515 filed Jun. 22, 2012, entitled “ANODIZATION,” U.S.Provisional Application Ser. No. 61/701,568 filed Sep. 14, 2012 entitled“ANODIZATION,” and U.S. Provisional Application Ser. No. 61/702,202filed Sep. 17, 2012 entitled “ANODIZATION,” each of which areincorporated herein by reference in their entirety.

FIELD OF THE DESCRIBED EMBODIMENTS

This disclosure relates generally to anodizing processes. Morespecifically, methods for producing an anodized film having a whiteappearance are disclosed.

BACKGROUND

Anodizing is an electrolytic passivation process used to increase thethickness of a natural oxide layer on a surface of metal parts where thepart to be treated forms the anode electrode of an electrical circuit.Anodizing increases corrosion resistance and wear resistance, canprovide better adhesion for paint primers and glues. The anodized filmcan also be used for a number of cosmetic effects. For example,techniques for colorizing anodized films have been developed that canprovide an anodized film with a perceived color based, in part, upon atype and amount of light reflection at the anodized film surface. Aparticular color can be perceived when a light of a specific frequencyis reflected off the surface of the anodized film.

In some cases, it can be desirable to form an anodized film having awhite color. However, conventional attempts to provide a white appearinganodized film have resulted in anodized films that appear to beoff-white, muted grey and milky white, and not a crisp and cleanappearing white that many people find appealing.

SUMMARY

This paper describes various embodiments that relate to metal oxidefilms and methods for forming the same. Embodiments presented hereindescribe white appearing metal oxide films and methods for forming thesame.

According to one embodiment, a method is described. The method involvessequentially varying a current density while forming a layer of aluminumoxide on an aluminum substrate. The layer of aluminum oxide issubstantially opaque and reflects substantially all wavelengths of whitelight incident thereon.

According to another embodiment, a metal substrate is described. Themetal substrate has a protective film disposed over an underlying metalsurface. The protective film has a porous structure with a whiteappearance, the porous structure having a number of pores. At least aportion of the pores includes irregular pore walls having a number ofsequentially repeating wide portions and narrow portions. Thesequentially repeating wide portions and narrow portions provide anumber of visible light reflecting surfaces positioned at variousorientations with respect to a top surface of the protective film suchthat substantially all visible wavelengths of light incident the topsurface diffusely reflect from the visible light reflecting surfaces andexit the top surface.

According to an additional embodiment, a method for forming micro-crackswithin a porous structure of an anodized film such that the anodizedfilm appears white is described. The method includes forming a patternof melted portions within the porous structure by scanning a pulsedlaser beam over a top surface of the anodized film. The method alsoincludes forming a pattern of crystallized metal oxide portions withinthe anodized film by allowing the pattern of melted portions to cool andtransform into crystalline form. During the cooling, a number ofmicro-cracks form within the pattern of crystallized metal oxideportions. The micro-cracks diffusely reflect nearly all visiblewavelengths of light incident the crystallized metal oxide portions.

According to a further embodiment, a metal part having an anodized filmwith a white appearance disposed over an underlying surface of the metalpart is described. The anodized film includes a porous metal oxidestructure. The anodized film also includes a pattern of crystallizedmetal oxide portions within the porous metal oxide structure, thepattern of crystallized metal oxide portions having a number ofmicro-cracks. The micro-cracks have a plurality of visible lightreflecting surfaces arranged in varied orientation with respect to anexposed surface of the anodized film. The visible light reflectingsurfaces diffusely reflect visible light incident the crystallized metaloxide portions, contributing an opaque and white appearance to the metalpart.

According to another embodiment, a method for forming an anodized filmon a substrate is described. The method includes sputtering a layer ofaluminum onto a substrate, the sputtered aluminum layer having a surfacewith a first roughness. The method also includes converting a firstportion of the sputtered aluminum layer to an anodized film. Anunderlying second portion of the sputtered aluminum layer has a secondsurface that has a second roughness associated with the first roughness.The second surface is sufficiently rough such that white light incidentto an exposed surface of the anodized layer travels through the anodizedlayer, diffusely reflects off the second surface, and exits the anodizedlayer.

According to an additional embodiment, a method for producing ananodized film that appears white is described. The method involvescreating a number of openings within the anodized film. The openingshaving an average size and shape suitable for accommodating a number oflight reflective particles. The light reflective particles have a whiteappearance due to the presence of multiple visible light diffusingsurfaces on the light reflective particles. The method also involvesinfusing the light reflective particles within at least a portion of theopenings. The white appearance of the light reflective particles impartsa white appearance to the anodized film.

BRIEF DESCRIPTION OF THE DRAWINGS

The described embodiments and the advantages thereof may best beunderstood by reference to the following description taken inconjunction with the accompanying drawings. These drawings in no waylimit any changes in form and detail that may be made to the describedembodiments by one skilled in the art without departing from the spiritand scope of the described embodiments.

FIGS. 1A-1D illustrate various reflection mechanisms for providing aperceived color or quality of an object.

FIG. 2 illustrates a cross section view of a part with an anodized filmformed using standard anodizing conditions.

FIG. 3 illustrates a cross section view of a part with a white anodizedfilm formed using varied current densities.

FIGS. 4A and 4B show graphs indicating current density as a function oftime during two different varied current density anodizing processes.

FIG. 5 shows a graph indicating current density as a function of timeduring another varied current density anodizing process.

FIG. 6 shows a flowchart indicating steps for forming a white anodizedfilm having irregular or textured pore walls using a varied currentdensity anodizing process.

FIGS. 7A-7C illustrate top and cross section views of a part having awhite anodized film after undergoing a laser cracking procedure.

FIG. 8 shows a flowchart indicating steps for forming a white anodizedfilm having micro-cracks using a raster scanning pulsed laser beam.

FIGS. 9A-9C illustrate different laser scan samples with varying spotdensity, laser power and spot size settings.

FIG. 9D illustrates a graph showing specular reflected light intensityas a function of viewing angle for different anodized film samples.

FIG. 10 shows a flowchart indicating steps for tuning a laser crackingprocess for producing a white anodized film having a target amount ofdiffuse and specular reflectance.

FIG. 11 illustrates a cross section view of a part with a white anodizedfilm formed using a combination of varied current density anodizing andlaser cracking procedures.

FIG. 12 shows a flowchart indicating steps for forming a white anodizedfilm formed using a combination of varied current density anodizing andlaser cracking procedures.

FIGS. 13A-13B illustrate cross section views of a part undergoing areflective layer depositing process following by an anodizing process.

FIG. 14 shows a flow chart indicating steps for forming a white anodizedfilm by depositing an underlying reflective layer.

FIGS. 15A-15C illustrate cross section views of a part undergoing a poreinfusion process.

FIGS. 16A and 16B illustrate cross section views of a part undergoing amicro-crack infusion process.

FIGS. 17A-17D illustrate top-down and cross section views of a partundergoing laser drilling, anodizing and light reflective particleinfusion processes.

FIG. 18 illustrates a light reflecting particle pore infusion processusing an electrophoresis technique.

FIG. 19 shows a flow chart indicating steps for forming a white anodizedfilm by infusing light reflective particles within openings of theanodized film.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Representative applications of methods according to the presentapplication are described in this section. These examples are beingprovided solely to add context and aid in the understanding of thedescribed embodiments. It will thus be apparent to one skilled in theart that the described embodiments may be practiced without some or allof these specific details. In other instances, well known process stepshave not been described in detail in order to avoid unnecessarilyobscuring the described embodiments. Other applications are possible,such that the following examples should not be taken as limiting.

This application relates to various embodiments of methods and apparatusfor anodizing an aluminum surface in such a way that the resultinganodized film appears white. The white appearing anodized films are wellsuited for providing both protective and attractive surfaces to visibleportions of consumer products. For example, methods described herein canbe used for providing protective and cosmetically appealing exteriorportions of metal enclosures and casings for electronic devices, such asthose manufactured by Apple Inc., based in Cupertino, Calif.

In general, white is the color of objects that diffusely reflect nearlyall visible wavelengths of light. Thus, an anodized film can beperceived as white when nearly all visible wavelengths of light incidenta top surface of the anodized film are diffusely reflected. FIG. 1A,shows how incident light can be diffusely reflected off a surface andscattered in many directions. Diffuse reflection can be caused byincident light reflecting off of multi-faceted surfaces at a top surfaceor within an object. For example, facets of ice crystals that form asnowflake diffusely reflect incident light, rendering the snowflakewhite in appearance. This is in contrast to specular reflection (FIG.1B) where light is reflected in one direction, colored matte appearingobjects (FIG. 1C) where some wavelengths of light are absorbed and onlycertain wavelengths of light are diffusely reflected, and black objects(FIG. 1D) where substantially all the wavelengths of light are absorbedand no light is reflected.

In the described embodiments, techniques involve forming white appearinganodized films. In some embodiments, the anodized film appears white dueto a combination of specular and diffuse reflection of all wavelengthspresent in white light due to structural features within the anodizedfilm. In some embodiments, the anodized film appears white due to thepresence of embedded particles that essentially “dye” the anodized filmwhite. In some embodiments, the anodized film appears white due to thepresence of an underlying light diffusing and reflecting layer. In somecases, two or more described techniques for producing white appearinganodized films can be combined.

The amount of perceived whiteness of an anodized film can be measuredusing any of a number of color analysis techniques. For example a coloropponent color space, such as L,a,b (Lab) color space (L indicates theamount of lightness, and a and b indicate color-opponent dimensions) canbe used to as a standard from which an objective determination of theperceived whiteness of different anodized film samples can be made. Insome embodiments described herein, optimum white anodized films have anL value ranging from about 85 to 100 and a,b values of nearly 0.Therefore, these anodized films are bright and color-neutral.

As used herein, the terms anodized film, anodized layer, anodizationfilm, anodization layer, oxide layer, and oxide film may be usedinterchangeably and can refer to any appropriate metal oxide film. Theanodized films are formed on metal surfaces of a metal substrate. Themetal substrate can include any of a number of suitable metals. In someembodiments, the metal substrate includes pure aluminum or aluminumalloy. In some embodiments, suitable aluminum alloys include 1000, 2000,5000, 6000, and 7000 series aluminum alloys.

Modifying Pore Walls

One method for forming a white appearing anodized film involves formingirregular pore walls during the anodizing process. FIG. 2 illustrates across section view of part 200 with anodized film 202 formed usingstandard anodizing conditions. During a standard anodizing process, atop portion of metal substrate 204 is converted to a layer of metaloxide, or anodized film 202, forming multiple self-organizing pores 206within anodized film 202. Pores 206 are elongated nanometer scale voidsthat are open at top surface 210 and that are defined by pore walls 208.As shown, pores 206 are highly ordered in that they are each arranged inperpendicular orientation with respect to top surface 210, and areequidistant and in parallel orientation with respect to each other.

Anodized film 202 is generally translucent in appearance since much ofthe incident white light coming in from top surface 210 can transmitthrough anodized film 202 and reflect off of at top surface ofunderlying substrate 204. For example, light ray 212 can enter from topsurface 210, pass through anodized film 202, reflect off of a surface ofunderlying substrate 204, pass again through anodized film 202, and exitat top surface 210. Since pore walls 208 are generally smooth anduniform, they do not substantially interfere with the transmission oflight ray 212 through anodized film 202. Thus, as viewed by an observerfrom top surface 210, anodized film 202 appears translucent and a viewerwould see underlying substrate 204. Since substrate 204 would reflectlight of a particular wavelength or range of wavelengths, part 200 wouldappear to have a color close to the color of underlying substrate 204.If underlying substrate 204 is smooth and reflective, the incident lightcan specularly reflect off underlying substrate 204 (as in a mirror inwhich an angle of incidence is equal to an angle of reflection). Forexample, light ray 214 can specularly reflect off underlying substrate204 in the same direction as light ray 212, giving part 200 a shinyreflective look. It should be noted that anodized film 202 is generallytranslucent, and not completely transparent, since smaller amounts ofincident light will not completely pass through anodized film 202 tounderlying substrate 204.

Methods described herein can be used to form an anodized film that hasan opaque and white appearance as viewed from a top surface. FIG. 3illustrates a cross section view of part 300 with anodized film 302formed using anodizing techniques in accordance with describedembodiments. During the anodizing process, a top portion of metalsubstrate 304 is converted to a layer of metal oxide, or anodized film302. As shown, pores 306 have pore walls 308 that are irregular inshape. Irregular pore walls 308 have multiple tiny surfaces that can actas reflection points for incident light. For example, light ray 312 canenter from top surface 310, pass through a portion of anodized film 302,reflect off of a first surface of irregular pore walls 308, pass throughanother portion of anodized film 302, and exit at top surface 310.Similarly, light ray 314 can enter from top surface 310, pass through aportion of anodized film 302, reflect off of a second surface ofirregular pore walls 308, pass through another portion of anodized film302, and exit at top surface 310. Since light rays 312 and 314 do notreach substrate 304, anodized film 302 is not transparent, i.e., opaque.That is, a viewer observing from top surface 310 would not be able tosee underlying substrate 304.

In addition to being opaque, anodized film 302 also has a whiteappearance. As described above, objects appear white when they diffuselyreflect, or scatter, nearly all visible wavelengths of light. Themultiple surfaces of irregular pore walls 308 arranged in varied anglescan scatter incident visible light at multiple different angles. Forexample, light ray 312 reflecting off the first surface of pore walls308 exits at top surface 310 at a first angle, while light ray 314coming in at the same angle as light ray 312 reflects off the secondsurface of pore walls 308 exits at top surface 310 at a second angledifferent from the first angle. Since irregular pore walls 308 have manysurfaces arranged in many different angles relative to top surface 310and each other, different light rays entering anodized film 302 at thesame angle will exit anodized film 302 at many different angles. In thisway, incident visible light can be diffusely reflected and impart awhite appearance to anodized film 302.

Techniques for forming a white anodized film with irregular pore walls,such as anodized film 302, include performing an anodizing process whileapplying a pulsed current density. In general, the current density canaffect the width of the pores, with higher current densities generallyforming wider pores and lower current densities generally formingnarrower pores. By varying the current density during pore growth, thepores are wide in some portions and narrow in other portions. Forexample, pores 306 can have wide portions having a first diameter 316formed during high current density conditions and narrow portions havinga second diameter 318 formed during low current density conditions,thereby forming irregular pore walls 308.

FIG. 4A shows graph 400 indicating current density (e.g., A/dm²) as afunction of time (e.g., minutes) during an anodizing process with variedcurrent density, in accordance with some embodiments. During theanodizing process, a substrate is placed in an anodizing solution andacts as anode when a voltage is applied. As the anodization processconverts part of the substrate to a metal oxide, the voltage isincreased to a high current density B and decreased to a low currentdensity A at different intervals. As shown, during time interval a, thecurrent density is ramped up from 0 to high current density B. Thecurrent density is maintained at high current density B for timeinterval b. During time interval b, the widths of the pores formingwithin the anodized film are relatively wide. During time interval c,the current density is decreased to low current density A. The currentdensity is maintained at low current density A for time interval d.During time interval d, the pores continue to form but have narrowerwidths relative to pore formation during time interval b. In someembodiments, time intervals a, b, c and d are on the order of minutes.The current density is then pulsed, i.e., increased to high currentdensity B and decreased to low current density A, for a series of timesuntil the anodized film reaches a target thickness and the anodizingprocess is complete. In this way, the widths of the pores can vary asthey are being formed, creating irregular pore walls, such as pore walls308 of FIG. 3.

FIG. 4B shows graph 420 similar to graph 400 of FIG. 4A, but withnon-linear increases and decreases in the current density. For example,during time interval a, the current density is ramped up from 0 to highcurrent density B in a non-linear fashion. Likewise, during timeinterval c, the current density is decreased to low current density A ina non-linear fashion. The manner in which the current density isincreased and decreased can affect the shape of the pore walls in theresultant anodized film.

The relative time periods of intervals a, b, c, and d presented ingraphs 400 and 420 are merely illustrative of particular embodiments anddo not necessarily dictate the relative time periods of otherembodiments. For instance, time intervals b can be shorter relative toa, c, and d, thereby applying very short pulses of high current density.In other embodiments, one or more time intervals a, b, c, and d are thesame. FIG. 5 shows graph 500 indicating current density (e.g., A/dm²) asa function of time (e.g., minutes) during an anodizing process withevenly spaced short pulses of high current density, in accordance withadditional embodiments. As shown, during time interval a, the currentdensity is ramped up from 0 to high current density B. The currentdensity is maintained at high current density B for time interval b.During time interval b, the widths of the pores forming within theanodized film are relatively wide. During another time interval b, thecurrent density is decreased to low current density A. The currentdensity is maintained at low current density A for an additional timeinterval b, during which time the pores continue to form but havenarrower widths relative to pore formation during high current densityB. In some embodiments, time interval b is on the order of minutes. Inother embodiments, time interval b is on the order of seconds. Thecurrent density is then pulsed, i.e., increased to high current densityB and decreased to low current density A, for a series of times untilthe anodized film reaches a target thickness and the anodizing processis complete. In some embodiments, the anodizing process can involveapplying a series of very short pulses of high current density followedby a series of longer pulsed of high current density. These differentparameters can affect the shape and irregularity of the pore walls indifferent ways, producing slight variations of whiteness of theresulting anodized film.

The low and high current density values described above with referenceto FIGS. 4A, 4B, and 5 can vary depending upon the desired pore wallshape and on particular application requirements. In some embodiments,high current density B ranges between about 2.0 and 4.0 A/dm² and lowcurrent density A ranges between about 0.5 and 2.0 A/dm². Since theapplied current density is related to voltage, the process can also bevaried with respect to high and low voltage values. The target thicknessof the anodized film can also vary depending, in part, on particularapplication requirements. In some embodiments, the anodizing process isperformed until a target thickness of about 20 to 35 microns isachieved.

In addition to controlling the shape and irregularity of the pore walls,the pores density can be controlled during the anodizing process byadjusting the anodizing bath temperature. In general, the higher thebath temperature, the thinner the metal oxide material is formed betweenthe pores and the higher the pore density. The lower the bathtemperature, the thicker the metal oxide material is formed between thepores and the lower the pore density. Higher pore density is directlyassociated with the amount of pore walls that can act as reflectivesurface for incident light. Therefore, the higher the pore density, thehigher the amount of irregularly shaped pore walls and the more lightscattering medium provided for diffusing incident light. As such, higherbath temperatures generally produce whiter anodized film than lower bathtemperatures. However, other factors, such as durability of the anodizedfilm, should also be considered when choosing the bath temperature. Insome embodiments, an anodizing bath temperature of about 0° C. to about25° C. is used.

FIG. 6 shows flowchart 600 indicating steps for forming a white anodizedfilm having irregular or textured pore walls using a varied currentdensity anodizing process, in accordance with some embodiments. At 602,the current density during an anodizing process is ramped up to a highcurrent density, such as high current density B of FIGS. 4 and 5. At604, the current density is maintained at the high current density for afirst time interval. During the first time interval, wide portions ofthe pores are formed. At 606, the current density is decreased to a lowcurrent density, such as low current density A of FIGS. 4 and 5. At 608,the current density is maintained at the low current density for asecond time interval. During the second time interval, narrow portionsof the pores are formed. Note that in some embodiments, the currentdensity is first ramped to the low current density, followed byincreasing to the higher current density. At 610, it is determinedwhether the target thickness of the anodized film is achieved. If thetarget thickness is achieved, the anodizing process is complete. If thetarget thickness has not yet been achieved, processes 604, 606, 608, and610 are repeated until the target thickness is achieved. In someembodiments, the target thickness is between about 5 and 50 microns. Insome embodiments, the target thickness is achieved at between about 20and 90 minutes. The resultant anodized film has pores with irregularpore walls that can diffusely reflect incident light, thereby impartinga white and opaque appearance to the anodized film.

Note that before and after the anodizing process of flowchart 600, oneor more of any suitable pre and post anodizing processes can beimplemented. For example, prior to anodizing, the substrate can undergoone or more cleaning, polishing and blasting operations. In addition,after anodizing, the anodized film can be colored using a dye orelectrochemical coloring process. In some embodiments, the surface ofthe anodized film is polished using mechanical methods such as buffingor lapping.

Forming Micro-Cracks within an Anodized Film

Another method for forming a white anodized film involves forminglocalized micro-cracks at the surface portions or sub-surface portionsof the anodized film. The cracks can be formed by raster scanning apulsed laser beam over a surface of the anodized film. FIGS. 7A and 7Billustrate a top view and a cross section view, respectively, of part700 after undergoing a laser cracking procedure, in accordance withdescribed embodiments. Part 700 includes anodized film 702 formed overunderlying substrate 704. During the laser cracking procedure, a pulsedlaser beam is raster scanned over top surface 710 of anodized film 702.The raster scanning produces a pattern of spot areas 714, whichrepresent areas of anodized film 702 that have been exposed to a pulseof a laser beam during the raster scanning. As shown, spot areas 714 arearranged in a pattern surrounded by unexposed areas 720. The size ofeach spot area 714 can be measured in terms of spot diameter 716 and canbe controlled by laser settings. Spacing 718 between spot areas 714 canbe controlled by controlling the raster settings of the laser apparatus.The raster scan pattern shown in FIGS. 7A and 7B are solely shown as anexample. In other embodiments, other raster scan patterns havingdifferent spacings 718 can be used. As shown, spot areas 714 penetrate adistance 717 within anodized film 702. Distance 717, in part, depends onthe wavelength of the laser beam. The laser beam should be a wavelengththat is tuned to interact with anodized film 702 without substantialinteraction with underlying substrate 704. In some embodiments, a CO₂laser is used, which produces infrared light having principle wavelengthbands centering around 9.4 and 10.6 micrometers.

Spot areas 714, which have been exposed to laser beam pulses, includemicro-cracks that can diffusely reflect incident light. To illustrate,FIG. 7C illustrates a close-up cross section view of part 700 showing aregion around a single spot area 714. As shown, areas 720 unexposed tothe laser beam have standard highly ordered pores 706 as part of aporous metal oxide structure. In contrast, the porous structure withinspot area 714 has been modified in the form of cracks 726. Cracks 726are formed when energy from the incident laser beam generates enoughlocalized heat that all or some portions of metal oxide material withinspot area 714 melt. That is, the heat is sufficient to at least reachthe glass transition temperature of the metal oxide material. When theheat dissipates and the metal oxide material cools, the metal oxidematerial transforms from an amorphous glass-like material to acrystalline form. In this way, the porous structure of the anodized film702 is transformed to a crystalline metal oxide form in spot areas 714.In addition, as the metal oxide cools, it contracts and causes cracks726 to form within spot area 714. In some embodiments, cracks 714 are onthe scale of between about 0.5 and 30 microns in length. Cracks 714 haveirregular interfaces that cause incident light to scatter. For example,light ray 722 reflects off of a first surface of cracks 726 at a firstangle, while light ray 724 coming in at the same angle as light ray 722reflects off a second surface of cracks 726 at a second angle differentfrom the first angle. Since cracks 726 have many surfaces arranged atmany different angles relative to top surface 710, different light rayswill reflect off cracks 726 at many different angles. In this way,incident visible light can be diffusely reflected off spot areas 714 andimpart a white appearance to anodized film 702.

FIG. 8 shows flowchart 800 indicating steps for forming a white anodizedfilm having micro-cracks using a raster scanning pulsed laser beam, inaccordance with some embodiments. At 802, an anodized film having aporous structure is formed on a substrate. As described above, astandard anodized film having a highly ordered porous structure can beused. At 804, portions of the porous structure are melted using a rasterscanning pulsed laser beam. The portions of the porous structure can bearranged in a raster pattern, such as shown in FIGS. 7A-7C, with eachspot area corresponding to a pulse of the laser beam. The laser beamshould be tuned such that the energy beam is focused on the anodizedfilm and not on the underlying substrate. At 806, the melted portions ofthe porous structure are allowed to cool and contract, thereby formingmicro-cracks within the porous structure. During the cooling process allor some of the melted portions can reform into crystalline metal oxideform. The resultant anodized film has micro-cracks that can diffuselyreflect incident light, thereby imparting a white and opaque appearanceto the anodized film.

In some embodiments, a combination of diffuse and specular reflectioncan be cosmetically beneficial. As described above, specular reflectionis when incident light is reflected in substantially one direction,imparting a mirror-like and shiny quality to an object. Specularreflection occurs when incident light reflects off of smooth surfacessuch as glass or calm bodies of water. Specular reflection can also makean object appear bright since the light is directly reflected off thesmooth surface. Thus, an anodized film that diffusely reflects light, aswell as specularly reflects light, can have a white and bright quality.Returning to FIG. 7C, incident light can specularly reflect offunderlying substrate 704 of unexposed areas 720 if the surface of theunderlying substrate is smooth. For example, light ray 728 specularlyreflects off of underlying substrate 704 of unexposed area 720. Thus,the relative amount of diffuse and specular reflection of anodized film702 can be controlled by controlling the relative amount of anodizedfilm 702 exposed to an incident laser beam. The amount of laser beamexposure can be controlled by parameters such as spot density, laserpower and spot size.

FIGS. 9A-9C show different laser scan samples illustrating how varyingspot density, laser power and spot size can affect the amount ofrelative diffuse and specular reflection of white anodized films. FIG.9A shows the effect of varying the spot density, or the raster pattern,of a laser beam. The spot density can be measured as a function of spotdiameter D. At sample 902, the distance between the centers of the spotsis three times the diameter D of the spots. At sample 904, the distancebetween the centers of the spots is two times the diameter D of thespots. At sample 906, the distance between the centers of the spots isequal to the diameter D of the spots. At sample 908, the distancebetween the centers of the spots is half of the diameter D of the spots.The more distance between the spots, the greater specular reflectionrelative to diffuse reflection. Thus, sample 908 will diffusely reflectmore light than sample 902. Sample 908 will have more of a white mattequality and sample 902 will have more of a reflective mirror-likequality.

FIG. 9B shows the effect of varying the laser power of a laser beam, asindicated by spot darkness. The laser power was varied from low laserpower at sample 910 and increased to high laser power at sample 916. Thehigher the laser power, the more diffuse reflectance will occur. Thus,sample 916 will have a more matte quality than sample 910. FIG. 9C showsthe effect of varying the spot diameters, or laser beam size, of theincident laser beam. Like the sample of FIG. 9A, samples 918, 920, 922and 924 each have different spot densities. However, the spot diametersof these samples are 40% smaller than the spot diameters of FIG. 9A.Samples 918, 920, 922 and 924 have different amounts of diffuse versusspecular reflective qualities compared to samples 902, 904, 906 and 908.

The amount of specular reflection of a white anodized film can bemeasured using any of a number of light reflection measurementtechniques. In some embodiments, a spectrometer configured to measurespecular light intensity at specified angles can be used. The measure ofspecular light intensity is associated with an amount of lightness and Lvalue, as described above. FIG. 9D shows graph 930 indicating specularreflected light intensity as a function of viewing angle for fourdifferent anodized film samples using a spectrometer. Each sample canhave a different spot area pattern, such as each of samples 902-924 ofFIGS. 9A-9C. Spectra 932, 934, 936 and 938 are from four differentsamples of anodized films taken at a 45 degree viewing angle. Spectrum936 corresponds to a target anodized film sample that has a desiredamount of specular reflection for producing a desired white and brightappearance. As shown, spectra 932 and 934 indicate samples that havegreater than target amount of specular reflection. Conversely, spectrum938 indicates a sample that has a lower than target amount of specularreflection. Thus, the spot density, laser power and spot size can betuned by measuring and comparing the amounts of specular reflection ofdifferent samples in order to produce a white anodized film having adesired amount of diffuse and specular reflection.

FIG. 10 shows flowchart 1000 indicating steps for tuning a lasercracking process for producing a white anodized film having a targetamount of diffuse and specular reflectance. At 1002, a white anodizedfilm using a laser cracking process is formed. The laser crackingprocess will have a set of parameters such as spot density, laser powerand spot size. At 1004, the amount of specular reflectance of the whiteanodized film is measured using a spectrometer. As described above, thespectrometer can measure the spectral reflectance at a defined angle andgenerate a corresponding spectrum. At 1006, the specular reflectancespectrum of the white anodized film is compared to a target specularreflectance spectrum. The target specular reflectance spectrum willcorrespond to a white anodized film having a desired amount of specularand diffuse reflection.

At 1008, it is determined from the comparison whether the amount ofspecular reflectance of the white anodized film is too high. If thespecular reflectance is too high, at 1010, the relative amount ofdiffuse reflectance is increased by changing process parameters, such asby increasing the spot density and/or laser power. Then, returning to1002, an additional white anodized film is formed using a laser crackingprocess with the new process parameters. If the specular reflectance isnot too high, at 1012, it is determined from the comparison whether theamount of specular reflectance of the white anodized film is too low. Ifthe specular reflectance is too low, at 1014, the relative amount ofdiffuse reflectance is decreased by changing process parameters, such asby decreasing the spot density and/or laser power. Then, returning to1002, an additional white anodized film is formed using a laser crackingprocess with the new process parameters. If the specular reflectance isnot too low, the white anodized film has a target amount of diffuse andspecular reflectance.

In some cases, it can be desirable to produce a white anodized filmhaving both light diffusing irregular pores, as described above withreference to FIGS. 3-6, and light diffusing cracks, as described abovewith reference to FIGS. 7-10. FIG. 11 illustrates a cross section viewof part 1100 with anodized film 1102 formed using anodizing techniquesin accordance with described embodiments. During an anodizing process, atop portion of metal substrate 1104 is converted to anodized film 1102.Also during the anodizing process, the current density is varied, orpulsed, with a series of low and high current densities. The pulsedcurrent density during pore formation produces pores 1106 havingirregular pore walls 1108. Irregular pore walls 1108 have multiple tinysurfaces that are arranged a varied angles relative to top surface 1110that can act as reflection points for diffusing incident light. Forexample, light ray 1112 reflects off of a first surface of irregularpore walls 1108 at a first angle, while light ray 1113 reflects off asecond surface of irregular pore walls 1108 at a second angle differentfrom the first angle. Since irregular pore walls 1108 have many surfacesarranged at many different angles relative to top surface 1110,different light rays will reflect off irregular pore walls 1108 at manydifferent angles, thereby imparting an opaque and white quality toanodized film 1102.

In addition, after anodized film 1102 having irregular pore walls 1108is formed, anodized film 1102 has undergone a laser cracking procedure.During the laser cracking procedure, a pulsed laser beam is rasterscanned over top surface 1110 of anodized film 1102. Spot area 1114represents an area of anodized film 1102 that has been exposed to apulse from a laser beam during the raster scanning. Spot area 1114 hascracks 1126 that can diffusely reflect incident light. For example,light ray 1122 reflects off of a first surface of cracks 1126 at a firstangle, while light ray 1124 reflects off a second surface of cracks 1126at a second angle different from the first angle. Since cracks 1126 havemany surfaces arranged at many different angles relative to top surface1110, different light rays will reflect off cracks 1126 at manydifferent angles. In this way, cracks 1126 of spot areas 1114 contributea cosmetically appealing white and opaque quality to part 1100.

FIG. 12 shows flowchart 1200 indicating steps for forming a whiteanodized film formed using a combination of varied current densityanodizing and laser cracking procedures. At 1202, an anodized filmhaving irregular pore walls is formed by using a varied currentanodizing process. Incident visible light will diffusely reflect off theirregular pore walls and contribute an opaque and white quality toanodized film. At 1204, cracks are formed within portions of theanodized film using a laser cracking procedure. Incident visible lightwill diffusely reflect off the cracks and contribute an opaque and whitequality to the anodized film.

Adding an Underlying Light Diffusing Layer

One method for forming a white anodized film involves depositing a layerof white and reflective material below an anodized film such thatincident light shining through the anodized layer is diffusely andspecularly reflected back through the anodized layer and exits a topsurface. FIGS. 13A-13B illustrate cross section views of part 1300undergoing a reflective layer depositing process and an anodizingprocess in accordance with described embodiments. At FIG. 13A, aluminumlayer 1302 is deposited on metal substrate 1304. Aluminum layer 1302 canbe a substantially pure aluminum layer since pure aluminum is generallybrighter in color, i.e., spectrally reflective, compared to aluminumalloys. In some embodiments, aluminum layer 1302 can be deposited usinga plating process. In other embodiments, aluminum layer 1302 isdeposited using a physical vapor deposition (PVD) process. Aluminumlayer 1302 has a first rough surface 1306 that diffusely reflectsincident visible light. The PVD process can be tuned to provide theright amount of roughness 1306 to create a target amount diffusereflection. Aluminum layer 1302, as viewed from top surface 1308, canhave a silver metallic look of aluminum that has a whitened element fromrough surface 1306.

At FIG. 13B, a portion of aluminum layer 1302 is converted to analuminum oxide layer 1310. As shown, a portion 1303 of aluminum layer1302 remains beneath aluminum oxide layer 1310. Aluminum portion 1303has a second rough surface 1307 situated at interface 1316 betweenaluminum portion 1303 and aluminum oxide layer 1310. Second roughsurface 1307 is associated with and has similar dimensions as firstrough surface 1306 prior to anodizing. Thus, second rough surface 1307can also diffusely reflect light. In some embodiments, aluminum oxidelayer 1310 is translucent. Therefore, light incident to top surface 1308of aluminum oxide layer 1310 can travel through aluminum oxide layer1310 and diffusely reflect off second rough surface 1307, imparting awhite appearance to part 1300. For example, light ray 1312 can enteraluminum oxide layer 1310, reflect off a first surface of rough surface1306, and exit aluminum oxide layer 1310 at a first angle. Light ray1314 can enter aluminum oxide layer 1310 at the same angle as light ray1312, reflect off a second surface of rough surface 1306, and exitaluminum oxide layer 1310 at a second angle different from the firstangle.

In addition to surface roughness 1306, light diffusing qualities ofaluminum layer 1302 can be enhanced by varying the thickness of aluminumlayer 1302. Specifically, as the thickness of aluminum layer 1302 isincreased from 0 microns to 50 microns, the amount of spectralreflection produced by aluminum layer 1302 decreases and the amount ofdiffuse reflection increases. It is believed that this is due to therougher surface produced by the thicker sputtered on aluminum material.In general, the longer the sputtering time, the thicker aluminum layer1302 becomes. As described above, it can be cosmetically beneficial tohave a combination of spectral and diffuse reflection in order toprovide a white appearing surface that is also bright. In someembodiments, an aluminum layer 1302 having a thickness of ranging fromabout 10 and 25 microns produces a combination of diffuse and spectralreflection that is cosmetically white and bright.

FIG. 14 shows flow chart 1400 indicating steps for forming a whiteappearing anodized film on a substrate by depositing an underlyingreflective layer. At 1402, an aluminum layer having a sufficiently roughsurface to diffusely reflect incident light is deposited on thesubstrate. In some embodiments, the aluminum layer is substantially purealuminum. In some embodiments, the aluminum layer is sputtered onto thesubstrate. The roughness, and therefore the relative amount of diffuseversus spectral reflection, of the surface of the aluminum layer can betuned by controlling the type of sputtering and thickness of which thealuminum layer is sputtered on. At 1404, a portion of the aluminum layeris converted to an aluminum oxide layer. Since a portion of the aluminumlayer is converted, an underlying portion of the aluminum layer remainsbeneath the aluminum oxide layer. The underlying portion of the aluminumlayer as a second rough surface at the interface between the remainingaluminum layer and the aluminum oxide layer. The second rough surface isassociated with the first rough surface of the aluminum layer prior toanodizing. White light entering the aluminum oxide layer can travelthrough the aluminum oxide layer, diffusely reflect off the second roughsurface, and exit the aluminum oxide layer, thereby imparting a whiteappearance to the substrate.

Infusing Light Reflective Particles

An additional method for forming a white appearing anodized filminvolves infusing light reflective white particles within small openingsof the anodized film such that the anodized film takes on a whiteappearance. In some cases, the openings are anodic pores that arenaturally formed within the anodized film during the anodizing process.In other cases, the openings are created within the anodized film using,for example, a laser cracking process or a laser drilling process.

The light reflective particles can be any suitable particles that havemultiple visible light reflecting surfaces for diffusely and specularlyreflect substantially all wavelengths of visible light and to give thelight reflective particles a white color. In some embodiments, alumina(Al₂O₃) or titania (TiO₂), or a combination of alumina and titania, areused. The average size of the light reflective particles can dependpartially on the size of the openings in which the light reflectiveparticles are infused within. For example, larger particles may not beable to fit within small opening, in which case, smaller particles areused. The light diffusing particles should also be of a size thatoptimally diffusely and specularly reflects visible light. In oneembodiment using titania particles, an average particle diameter in therange of about 150 to 350 nanometers is used.

FIGS. 15A-15C illustrate cross section views of part 1500 undergoing apore infusion process, in accordance with some embodiments. At 15A, part1500 has undergone an anodizing process to convert a portion of metalsubstrate 1504 to anodized layer 1502. Pores 1506 form naturally duringthe anodizing process in elongated shapes with top ends opened atsurface 1510 and bottom ends proximate to underlying substrate 1504. Theaverage diameter 1508 of pores 1506 for a typical anodizing film rangesfrom about 10 to 130 nanometers, depending on the electrolyte used. At15B, pores 1506 are optionally widened to a larger average diameter1512. In some embodiments, pores 1506 are widened to average diameter1512 of greater than about 100 nanometers, in some cases to around 150nanometers or more. Any suitable pore widening process can be used. Forexample, subjecting part 1500 to an acidic solution can widen pores1506.

At 15C, pores 1506 are partially or completely filled with lightreflective particles 1514. The infusing of pores 1506 with lightreflective particles 1514 can be accomplished using any of a number ofsuitable techniques. For example, a sedimentation process, a pressingprocess, an electrophoresis process, or a PVD process can be used, whichare described in detail below. After pores 1506 are partially orcompletely filled, they are optionally sealing using any suitable poresealing process. Since light reflective particles 1514 are white bydiffusely reflecting visible light, they can impart white appearance toanodized layer 1506. For example, light ray 1516 reflecting off a firstsurface of light reflective particles 1514 exits at top surface 1510 ata first angle, while light ray 1518 coming in at the same angle as lightray 1516 reflects off a second surface of light reflective particles1514 and exits at top surface 1510 at a second angle different from thefirst angle. In addition, any bright specular reflective qualities thatlight reflective particles 1514 possess are also maintained while withinpores 1506, giving anodized layer 1506 a bright white appearance.

FIGS. 16A and 16B illustrate cross section views of part 1600 undergoinga micro-crack infusion process, in accordance with some embodiments. At16A, part 1600 has undergone a laser cracking procedure, such as thelaser cracking procedures described above with reference to FIGS. 7-12.As shown, pores 1606 of anodized layer 1602, situated over underlyingsubstrate 1604, have been modified within spot area 1614. Spot area 1614corresponds to an area exposed to a pulse of a laser beam. Micro-cracks1626 are formed as a result of localized heating from the laser beam andsubsequent cooling of the aluminum oxide material within spot area 1614.In some embodiments, micro-cracks have an average width 1627 rangingfrom about 100 nanometers to about 600 nanometers.

At FIG. 16B, light reflective particles 1628 are infused within cracks1626 using any of a number of suitable techniques, such as thosedescribed below. Since width of micro-cracks 1626 can be larger than theaverage diameter of typical pores, the size of light reflectiveparticles 1628 can be larger than those used in the pore infusionembodiment described above with reference to FIGS. 15A-15C. Lightreflective particles 1628 diffusely reflect light, imparting a whiteappearance to anodized layer 1602. For example, light rays 1622 and 1624reflect off a first surface and a second surface, respectively, of lightreflective particles 1628 at a first angle and a second angle,respectively. In addition, any bright specular reflective qualities thatlight reflective particles 1628 possess can contribute a bright specularquality to anodized layer 1606.

FIGS. 17A-17D illustrate top-down and cross section views of part 1700undergoing laser drilling and light reflective particle infusionprocesses, in accordance with some embodiments. FIG. 17A shows atop-down view of part 1700 with metal substrate 1704 having undergone alaser drilling process, whereby directing a laser beam at metalsubstrate 1704 produces an array of holes 1706. In some embodiments, apulsed laser system is used where each laser beam pulse corresponds toeach hole 1706. In other embodiments, multiple pulses of a laser beamform each hole 1706. In some embodiments, a pulsed laser beam is rasterscanned over substrate 1704. Holes 1706 can be arranged in an orderedarray, such as shown in FIG. 17A, or in a random pattern where holes1706 are randomly distributed within metal substrate 1704. In someembodiments, holes 1706 have an average diameter 1710 ranging from about1 micron to about 20 microns. Suitable pitch 1712 between holes 1706 canalso be selected. In some embodiments pitch 1712 can be on the scale ofaverage hole diameter 1710. Any suitable laser of producing a laser beamhaving a power and wavelength range for drilling holes within metalsubstrate 1704 can be used. FIG. 17B illustrates a close-up crosssection view of holes 1706 within metal substrate 1704. Depth 1714 ofopenings 1706 can vary depending on particular applications.

At FIG. 17C, part 1700 has undergone an anodizing process whereby aportion of metal substrate 1704 is converted to anodized layer 1702. Insome embodiments, anodized layer 1702 has a thickness 1716 ranging fromabout 15 microns to about 35 microns, depending on applicationrequirements. As shown, anodized layer 1702 substantially conforms tothe shape of metal substrate 1704 such that holes 1706 having a size anda shape appropriate for accommodating light reflective particles existwithin anodized layer 1702. At FIG. 17D, holes 1706 are partially orcompletely infused with light reflective particles 1718 using any of anumber of suitable techniques, such as those described below. Lightreflective particles 1718 diffusely reflect light, imparting a whiteappearance to anodized layer 1702. For example, light rays 1720 and 1722reflect off a first surface and a second surface, respectively, of lightreflective particles 1718 at a first angle and a second angle,respectively. In addition, any bright specular reflective qualities thatlight reflective particles 1718 possess can contribute a bright specularquality to anodized layer 1702.

As described above, a number of suitable techniques can be used toinfuse light reflective particles within openings, such as pores, cracksand laser drilled holes, within an anodized film. One technique forinfusing light reflective particles within openings of an anodized filminvolves a sedimentation process, whereby the force of gravity moves thelight reflective particles within the openings. The sedimentationtechnique involves placing the substrate into a slurry containing thelight reflective particles. The force of gravity sinks the lightreflective particles into the bottom of the openings of the anodizedfilm. The slurry is then heated to allow the liquid portion of theslurry to evaporate, leaving the light reflective particles within theopenings. In another variation, prior to exposing the substrate to theslurry, a vacuum desiccator is used to vacuum out air and create avacuum pressure within the openings where the light reflective particleswill be drawn into.

Another technique for infusing light reflective particles withinopenings of an anodized film involves a pressing technique, whereby thelight reflective particles are physically forced within the openings. Inone embodiment, a substrate is placed into a slurry containing the lightreflective particles. A fixture, such as a rubber roller, is then usedto press the light reflective particles into the openings of theanodized film. Next, the liquid portion of the slurry is allowed toevaporate, leaving the light reflective particles within the openings.As with the sedimentation technique described above, a vacuum enhancedvariation can be applied, whereby the substrate is placed in a vacuumdesiccator prior to exposure to the slurry and the pressing operation.

An additional technique for infusing light reflective particles withinopenings of an anodized film involves an electrophoresis technique,whereby the light reflective particles are attracted within the openingsby electrophoresis. FIG. 18 shows electrolytic assembly 1800illustrating an electrophoresis process whereby a DC voltage is appliedacross negatively charged cathode 1802 and positively charged anode1804, creating an electric field within electrolytic bath 1808. In thiscase, cathode 1802 acts as a substrate. Light reflective particles 1806are added to electrolytic bath 1808 and take on a positive charge,opposite cathode substrate 1802. As such, light reflective particles1806 migrate though electrolytic bath 1808 toward cathode substrate 1802and within any openings within the surface of cathode substrate. Whenthe voltage is removed, the light reflective particles remain within theopenings. Note that in other embodiments, the anode can act as thesubstrate, with negatively charged light particles attracted to thepositive anode substrate. In one embodiment, the light reflectiveparticles are titania (TiO₂), which can take on a positive charge withinan electrolytic solution, and are attracted to a cathode substrate.

Another technique for infusing light reflective particles withinopenings of an anodized film involves a PVD technique, whereby the lightreflective particles are sputtered onto the substrate. When the lightreflective particles are sputtered onto the substrate, some of the lightreflective particles become embedded within the openings. After the PVDprocess is complete, a separate process for removing excess portions oflight reflective material, i.e., material deposited at surface, can beremoved, thereby leaving the openings filled with light reflectiveparticles.

FIG. 19 shows flowchart 1900 indicating steps for forming a whiteanodized film by infusing light reflective particles within openings ofthe anodized film. At 1902, openings are created within an anodizedfilm. In some embodiments, the openings are the pores that areconcurrently formed with growth of the anodized film. In otherembodiments, the openings are formed using a separate procedure, such asa laser cracking or a laser drilling procedure. The openings should besized and shaped suitable for accommodating light reflective particles.At 1904, light reflective particles are infused within the openings ofthe anodized film. Any suitable infusion technique can be used. Forexample, a sedimentation process, a pressing technique, anelectrophoresis technique, or a PVD technique described above can beused.

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the describedembodiments. However, it will be apparent to one skilled in the art thatthe specific details are not required in order to practice the describedembodiments. Thus, the foregoing descriptions of specific embodimentsare presented for purposes of illustration and description. They are notintended to be exhaustive or to limit the described embodiments to theprecise forms disclosed. It will be apparent to one of ordinary skill inthe art that many modifications and variations are possible in view ofthe above teachings.

What is claimed is:
 1. A method of modifying a part such that the part appears white, the part including an aluminum oxide layer on a substrate, the method comprising: modifying a structure of the aluminum oxide layer, wherein the structure of the aluminum oxide layer comprises a plurality of pores extending from an exterior surface of the aluminum oxide layer toward a reflective surface of the substrate, wherein the modifying comprises: scanning a laser beam across the exterior surface forming an array of substantially uniformly spaced light diffusing portions within the aluminum oxide layer, each of the light diffusing portions having a plurality of micro-cracks that diffusely reflect a first portion of light incident the exterior surface providing a diffusely reflective aspect, wherein an intervening portion of the aluminum oxide layer between the light diffusing portions allows a second portion of light incident the exterior surface to pass through the intervening portion such that the second portion of light reflects off the reflective surface providing a specularly reflective aspect, wherein the diffusely reflective aspect combines with the specularly reflective aspect to impart a white appearance to the part.
 2. The method of claim 1, wherein the intervening portion is substantially unaffected by the laser beam.
 3. The method of claim 1, wherein the some of the light diffusing portions overlap with each other.
 4. The method of claim 1, wherein after modifying the structure of the aluminum oxide layer, the part has an L value ranging between about 85 and
 100. 5. The method of claim 1, wherein the laser beam is from a pulsed carbon dioxide laser.
 6. The method of claim 1, further comprising infiltrating particles into at least some of the plurality of micro-cracks, the infiltrated particles arranged to diffusely reflect light incident the exterior surface contributing to the white appearance of the part.
 7. The method of claim 1, wherein an average diameter of the light diffusing portions and an average distance between the light diffusing portions are chosen so as to impart a predetermined light intensity to the part.
 8. The method of claim 7, wherein the average distance is about 3 times the average diameter or less.
 9. The method of claim 7, wherein the average distance is about the same as the average diameter or less.
 10. The method of claim 7, wherein the average distance is about half the average diameter.
 11. The method of claim 7, further comprising: determining whether a light intensity of the part is higher than or less than the predetermined light intensity, and when it is determined that the light intensity of the part is less than the predetermined light intensity, increasing the average distance between the light diffusing portions.
 12. The method of claim 7, further comprising: determining whether a light intensity of the part is higher than or less than the predetermined light intensity, and when it is determined that the light intensity of the part is higher than the predetermined light intensity, decreasing the average distance between the light diffusing portions.
 13. A method of modifying a part such that the part takes on a white appearance, the method comprising: modifying a metal oxide layer of the part, the metal oxide layer positioned on a reflective surface of an underlying substrate, the metal oxide layer including a plurality of anodic pores and an exterior surface, wherein the modifying comprises: forming an array of substantially uniformly spaced light diffusing portions within the metal oxide layer by scanning a laser beam across the exterior surface of the metal oxide layer, each of the light diffusing portions having a plurality of micro-cracks that diffusely reflect a first portion of light incident the exterior surface providing a diffusely reflective aspect, wherein an intervening portion of the metal oxide layer between the light diffusing portions allows a second portion of light incident the exterior surface to pass therethrough such that the second portion of light reflects off the reflective surface providing a specularly reflective aspect, wherein the diffusely reflective aspect combines with the specularly reflective aspect to impart a white appearance to the part.
 14. The method of claim 13, wherein the micro-cracks are on a scale of between 0.5 and 30 microns in length.
 15. The method of claim 13, wherein the part is characterized as having an L value ranging from 85 to
 100. 16. The method of claim 13, wherein the intervening portion is substantially free of micro-cracks.
 17. The method of claim 13, wherein an average diameter of the light diffusing portions and an average distance between the light diffusing portions are chosen so as to impart a predetermined light intensity to the part.
 18. The method of claim 17, wherein the average distance is about 3 times the average diameter or less.
 19. The method of claim 17, wherein the average distance is about half the average diameter.
 20. The method of claim 17, wherein the some of the light diffusing portions overlap with each other.
 21. The method of claim 17, wherein at least some of the plurality of micro-cracks includes particles infiltrated therein, the particles arranged to diffusely reflect light incident the exterior surface contributing to the white appearance of the part. 